This invention relates to cathodes for electrolytic cells for the production of aluminum, and specifically to the method for preparation of cathode plates or tiles of enhanced quality and increased service life for use in such cells. The cathode plate materials are subjected to vacuum mixing under conditions of low shear, cast, cured, and baked to form aluminum wettable Refractory Hard Material/carbon elements of higher density, strength, and durability than previously believed attainable.
Aluminum is conventionally manufactured by an electrolytic reduction process conducted in Hall-Heroult cells, wherein alumina is dissolved in molten cryolite and electrolyzed at temperatures of 900.degree.-1000.degree. C. These cells typically comprise a steel shell with an insulating lining of suitable refractory materials, which in turn is provided with a lining of carbon which contacts the molten bath, aluminum, and/or ledge. One or more anodes, usually made of carbon, are inserted into the molten cryolite and connected to the positive pole of a direct current source. The negative pole of the direct current source is connected to the carbon lining in the bottom of the cell. Molten aluminum resulting from the electrolytic reduction reaction is collected on the carbon bottom of the cell in a molten pool or pad, which acts as a liquid metal cathode onto which additional aluminum deposits. A portion of this pool of liquid is removed periodically and collected as the product of the electrolysis process.
In the construction of most modern commercial cells, the carbon lining that forms the top layer of the cathode is conventionally built from an array of prebaked carbon blocks covering the portion of the cell to be lined, and then the carbon blocks are joined into a solid continuous assembly by ramming the slots between blocks with a mixture typically of coke, calcined anthracite, modified coal tar pitch, and the like. This structure is then heated in the process of cell start-up. The life span of such carbon linings in different plants averages three to eight years, but under adverse conditions may be considerably shorter. Deterioration occurs due to penetration of molten electrolyte components and liquid aluminum into the structure of the carbon blocks, and ramming mix, causing swelling and cracking. Aluminum metal penetration causes alloying and slow destruction of the steel current collector bars embedded in the cell bottom. This contaminates the aluminum pad and may eventually lead to cell tap-out.
Other problems in conventional aluminum reduction cell operation include accumulation of undissolved or frozen bath and alumina which are carried from the cryolite bath, ledge, and ore cover, to the cathode, creating sludge or muck. The presence of this sludge or muck under the aluminum pad creates electrically insulated areas on the cell bottom which increase the cathode voltage loss and disrupt electrical current distribution, resulting in excessive pad turbulence and disturbances through magnetic forces, hence reducing cell current efficiency.
A further drawback of the carbon cathode lining is its non-wettability by molten aluminum, which necessitates operation with a deep pad of aluminum, to ensure effective molten aluminum contact to the carbon lining or surface. The deep aluminum pad is subject to magnetic and electrical interactions, such as standing waves, which increase the possibility of electrical shorting to the anode. To lessen this possibility, greater anode-to-cathode distances (ACD) are conventionally employed, resulting in additional voltage requirements.
To reduce ACD and associated voltage drop, it is necessary to make adjustments in magnetic design, or to operate without an aluminum pad. To achieve the latter goal, attempts have been made to use cathode materials comprising Refractory Hard Material (RHM), such as TiB.sub.2. Titanium diboride is highly conductive and is wetted by liquid aluminum. This wettability property enables a thin film of molten aluminum to be deposited directly on the cathode structure made of RHM, and eliminates the need for a pad of metal, since contact with the underlying cathode structure is assured.
The use of titanium diboride current-conducting elements in electrolytic cells for the production of aluminum is described in the following exemplary U.S. Pat. Nos: 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093, and 3,400,061. Despite the extensive effort expended in the past, as indicated by these and other patents, and the potential advantages of the use of titanium diboride as a current-conducting element, such compositions do not appear to have been commercially adopted on any significant scale by the aluminum industry. Lack of acceptance of TiB.sub.2 or RHM current-conducting elements of the prior art is related to their lack of durability in service in electrolytic reduction cells. It has been reported that such current-conducting elements fail after relatively short periods in operation. Such failure has been associated with the penetration of the self-bonded RHM structure by the electrolyte, and/or aluminum, thereby causing critical weakening with consequent loss of cohesion, cracking, and failure. It is well known that liquid phases penetrating the grain boundaries of solids can have undesirable effects. For example, prior art RHM tiles, in which oxygen impurities were found to segregate along grain boundaries, are susceptible to rapid attack by aluminum metal and/or cryolite bath. Prior art techniques to combat TiB.sub.2 tile disintegration in aluminum cells have included use of highly refined TiB.sub.2 powder to make tiles containing less than 50 ppm oxygen. However, this high purity material costs 3 or 4 times as much as commercially pure TiB.sub.2 powder (containing about 3000 ppm oxygen), and the necessary high temperature fabrication further increases the cost of TiB.sub.2 tiles substantially. Despite the use of high purity materials, no cell utilizing TiB.sub.2 tiles is known to have operated successfully for extended periods without loss of adhesion of the tiles to the cathode, or disintegration of the tiles. Other reasons proposed for failure of RHM tiles and coatings have been the solubility of the materials in molten aluminum or molten bath, the lack of mechanical strength, and the poor resistance to thermal shock.
Additionally, different types of TiB.sub.2 materials, applied to carbon substrates, have failed due to differential thermal expansion between the titanium diboride material and the carbon cathode block. To the Inventor's knowledge no prior RHM-containing tiles or plates, even of high purity, have been successfully operated as a commercially employed cathode structure or surface layer, because of thermal expansion mismatch, bonding problems, etc.
For example, U.S. Pat. No. 3,400,061, of Lewis et al, assigned to Kaiser Aluminum, teaches a cell construction with a drained and wetted cathode, wherein the Refractory Hard Material cathode surface consists of a mixture of Refractory Hard Material, at least 5 percent carbon, of which 10 to 20 percent by weight is derived from pitch, baked at 900.degree. C. or more. According to the patent, such a composite cathode has a higher degree of dimensional stability when electrolysed in a molten bath environment than previously attainable with carbon. The composite cathode coating material of this reference may be rammed into place in the cell bottom. Alternatively, shapes composed of the composite material may be produced in a separate facility for placement on a cathode block. Such material has not been widely adopted, however, due to susceptibility to attack by the electrolytic bath, as taught by a later Kaiser Aluminum U.S. Pat. No. 4,093,524 of Payne.
Said U.S. Pat. No. 4,093,524, of Payne, claims an improved method of bonding titanium diboride, and other Refractory Hard Materials, absent carbonaceous binders, to a conductive substrate such as graphite, or to silicon carbide. The cathode surface is made from titanium diboride tiles, 0.30 to 2.50 cm thick. Payne recognized that the large differences in thermal expansion coefficients between such Refractory Hard Material tiles and carbon precludes the formation of a bond which will be effective both at room temperature and at operating temperatures of the cell. The bonding is accordingly formed in-situ at the interface between the Refractory Hard Material tile and the cathode by a reaction between aluminum and the carbon beneath the tile to form aluminum carbide only when the cell approaches operating temperature. However, since the bond is not formed until high temperatures are reached, tiles are easily displaced during startup procedures. The bonding is accelerated by passing electrical current across the interface, resulting in a very thin aluminum carbide bond. However, electrolyte attack upon the bond results if the tiles are installed too far apart, or if the protective film of aluminum on the surface is incomplete. Alternatively, if the tiles are installed too close together, they bulge at operating temperature, resulting in rapid deterioration of the cell lining and in disturbance of cell operations. Further problems would probably be witnessed during fluctuations in cell temperature and during a shut-down and restart of a cell employing such bonding, because the thermal expansion mismatch has not been eliminated, merely circumvented at high temperature. Accordingly, this concept has not been extensively utilized.
Holliday, in U.S. Pat. No. 3,661,736, claims a dimensionally stable composite cathode for a drained and wetted cell, comprising particles or chunks of highly purified arc-melted "RHM alloy" embedded in an electrically conductive binder matrix, which may be carbonaceous. In this particular instance, the surface of the matrix becomes protected by an aluminum carbide layer. However, in operation of such a cell, electrolyte and/or aluminum attack the matrix material, large areas of which are exposed to contact, consequently leading to early destruction of the structure. Moreover, the relatively large chunks of TiB.sub.2 suffer from the same drawbacks, in terms of poor thermal shock resistance, brittleness, etc., as wholly RHM materials.
U.S. Pat. No. 4,308,114, of Das et al, discloses a contoured cathode surface composed of Refractory Hard Material in a fully graphitic matrix. In this case, the Refractory Hard Material is composited with a pitch binder, and subjected to graphitization at about 2350.degree. C., or above. Such cathodes are subject to early failure due to relatively rapid ablation, caused by physical erosion and aluminum carbide formation in the graphite matrix.
In addition to the above patents, a number of other references relate to the use of titanium diboride in tile form. Titanium diboride tiles of high purity and density have been tested, but they generally exhibit poor thermal shock and impact resistance and are difficult to bond to carbon substrates employed in conventional cells. Mechanisms of de-bonding are believed to involve high stresses generated by the thermal expansion mismatch between the titanium diboride and carbon, as well as aluminum penetration along the interface between the tiles and the adhesive holding the tiles in place, due to wetting of the bottom surface of the tile by molten aluminum. In addition to debonding, disintegration of even high purity tiles may occur due to aluminum penetration of grain boundaries. These problems, coupled with the high cost of the titanium diboride tiles, have discouraged extensive commercial use of titanium diboride elements in conventional electrolytic aluminum smelting cells, and limited their use in new cell design. To overcome the deficiencies of past attempts to utilize Refractory Hard Materials as a surface element for carbon cathode blocks, coating materials comprising Refractory Hard Materials in a carbonaceous matrix have been suggested.
In U.S. patent application Ser. Nos. 400,762 (pending), 400,772 (pending), and 400,773 (now U.S. Pat. No. 4,466,966), filed July 29, 1982 by Boxall et al, formulations, application methods, and cells employing TiB.sub.2 /carbon cathode coating materials were disclosed. This technology relates to spreading a mixture of Refractory Hard Material and carbon solids with thermosetting carbonaceous resin on the surface of a cathode block, followed by cure and bake cycles. Improved cell operations and energy savings result from the use of this cathode coating process in conventionally designed commercial aluminum reduction cells. Plant test data indicate that the energy savings attained and the coating life are sufficient to make this technology a commercially advantageous process.
Advantages of such composite coating formulations over hot pressed RHM tiles include much lower cost, less sensitivity to thermal shock, thermal expansion compatibility with the cathode block substrate, and less brittleness. In addition, oxide impurities are not a problem, and a good bond to the carbon cathode block may be formed which is uneffected by temperature fluctuations and cell shutdown and restart. Pilot plant and operating cell data indicate that a coating life of from four to six years or more may be anticipated, depending upon coating thickness.
However, problems inherent in this coating process include the fact that modifications in the coating formulation are required to compensate for changes in the mechanical properties and thermal expansion coefficients of different cathode blocks. The process is labor intensive and requires complex cure and bake heat treatments which can be very disruptive to plant operations. The need to maintain a good bond to the cathode block during the cure and bake heat treatments necessitates the use of less than optimum formulations and process conditions. Quality control is also difficult to maintain in the plant environment. Further, the variable electrical resistivity of the coating prior to baking can result in severe problems during cell start-up and finally, coating thickness is limited to approximately 1.2 cm.
Attempts have been made to retain the advantages of the novel composite coating material, as formulated and tested, while minimizing problems detailed above. For example, in U.S. patent application Ser. No. 461,893 (now U.S. Pat. No. 4,481,052), filed Jan. 28, 1983 by Buchta and Nagle, fabrication of hot pressed tiles is disclosed. However, the tiles prepared in accordance therewith, (utilizing thermosetting resin, Refractory Hard Material, and graphite, and formed under high pressure at elevated temperatures,) are mechanically soft, particularly after exposure to the aluminum cell environment, and are susceptible to aluminum carbide formation and consequent wear.
In contrast, the present invention retains the advantages of the novel composite coating material, without the drawbacks of the hot pressed tiles, by preparing structural shapes which are fabricated and heat treated prior to application to the cathode, rather than applying the material directly to the cathode substrate and then heating. Specifically, the added improvements include the fact that once baked, the plate material has a thermal expansion coefficient which is essentially equal to carbon; hence, only a simple carbon to carbon bond is required to attach the plate to the carbon cathode block. The production process can be readily automated at a central manufacturing plant. The baked plates can be glued to the cathode blocks either by the block manufacturer (or other vendor), or in the plant. Moreover, a central plate manufacturing plant affords the best equality control, since only a simple gluing operation is left for the less controlled smelter environment. Since there is no substrate-to-composite bond to maintain during the initial cure and bake heat treatments accompanying the material fabrication, the process can be optimized to produce the highest quality cathode plate material at a minimum cost. Once baked, the plate material is highly electrically conductive and therefore does not interfere with cell start up procedures. Multiple plates can be glued together to give any desired thickness. More efficient heat sources such as microwave can be used to cure the plate material, and complex shapes can be produced by this process to meet the needs of all foreseeable low energy cell designs.
In copending patent application Ser. No. 576,835 (abandoned, continued as Ser. No. 729,888), filed concurrently herewith, Boxall discloses molded aluminum wettable plates of Refractory Hard Material in a carbonaceous matrix bonded by amorphous carbon, derived from a thermosetting resin. Such a material is described in the aforementioned patent applications of Buchta et al, and Boxall et al. According to the invention described by Boxall, the composition is cast or pressed in a mold, then cured and baked to produce a rigid, electrically conductive element. The cathode plates may then be cemented to the aluminum smelting cell cathode substrate using conventional carbon cementing techniques. This method eliminates the need for compromising the coating composition in order, for example, to minimize differences in expansion behavior between block and coating during cure and heat-treatment prior to carbonization. After heat treatment and subsequent carbonization, the cast or pressed plates have a thermal expansion coefficient very close to that of carbon cathode blocks, so that maintaining attachment over wide temperature ranges is not a problem.
Boxall's disclosure demonstrates cathode plate elements and the ease of their attachment to cathode blocks or other similar carbon bodies. Electrical resistance measurements across the plate-to-substrate interface show that conventional start-up procedures will be fully viable for cells lined with the cathode plate material. It is well known that improvements in quality of carbon based cathode lining materials, such as increased density, reduced porosity, and freedom from flaws, result in improved performance in service, and increased service life. Porosity and flaws are known to increase susceptibility to thermal, chemical, and mechanical damage in the highly aggressive environment of an aluminum cell. The present invention reflects extensive development work undertaken to transfer Boxall's findings into a fully viable process for making such high quality plate material on a production basis.